This application relates to fully dense, reinforced ceramic composites whose improved strength and toughness make their mechanical and structural properties more desirable, and relates particularly to a fully dense alumina composite reinforced by zirconia-clad alumina fibers. The invention which is the subject matter herein results from our solution to the problem of alumina reinforcing fibers dissolving in the alumina matrix under conditions necessary to achieve densification, a problem which if addressed at all certainly has not been solved heretofore.
Ceramics normally exhibit very poor fracture toughness and thermal shock resistance which limits their use as structural components in, for example, heat engines and turbines. For example, alumina is a rather low cost ceramic which could be used far more extensively if it did not exhibit these characteristics. Over the last few years two general methods have been developed to toughen ceramics; transformation toughening and fiber toughening.
Transformation toughening arises from the addition of a second oxide, commonly zirconia, which shows a phase transformation during processing. In the case of zirconia, there is a high temperature tetragonal phase and a low temperature monoclinic phase of lower density. When this phase transition occurs during processsing, the volume expansion causes microcracks to form around zirconia particles in a ceramic matrix. A propagating crack will upon hitting this particle be split into several parts and the energy of the crack is thereby dissipated. Alternately, it is possible to retain metastable tetragonal phase zirconia particles in ceramic matrices. When a propagating crack approaches such a particle the large tensile stresses ahead of a crack tip causes the zirconia to undergo a phase transformation. The resulting volume expansion places the matrix in compression near the particle, stopping the propagating crack. See, for example, N. Claussen, J. Am. Ceram. Soc., 59 (1-2), 49 (1976).
Another method for increasing fracture toughness is by incorporation of high strength fibers into the ceramic matrix. It is believed that the fibers tend to retard crack propagation by absorbing energy when the fiber is pulled from the surrounding metal oxide matrix. Additionally, cracks may be deflected and branch when they hit the fiber-matrix interface. An example of this form of toughening is an alumina matrix reinforced with silicon carbide whiskers. G. C. Wei and P. F. Becker, Bull. Am. Ceram. Soc., 64 (2), 298 (1985); G. C. Wei, U.S. Pat. No. 4,543,345. However, there are several disadvantages associated with the use of silicon carbide whiskers as reinforcing fibers. One is that such whiskers currently are quite expensive. Another is that a number of physical properties, such as electrical and thermal conductivities, of silicon carbide are quite different than those of alumina, and since an appreciable volume fraction of reinforcing fibers (5-60 volume percent) may be used the physical properties of the resulting composite can be significantly different than those of alumina.
Alumina-based fibers are relatively well known and commercially available. These are relatively inexpensive in comparison with silicon carbide whiskers and have reasonably good mechanical properties. It is highly desirable to reinforce alumina with such fibers not only because of their low cost, but also because the physical properties of the resulting composite would be unchanged by the addition of such fibers. Attempts to prepare porous alumina composites containing high strength alumina fibers have been successful, as exemplified by European patent application 130-105. Applicants there disclose composites which include an alumina fiber reinforced alumina matrix, but an essential limitation is that the composite have at least 30% porosity. In the context of this application, porosity is 100.multidot.[1-(y/x)], where y is the measured density, and x is the theoretical full density. The resulting composites are not strong, tough ceramics which retain these structural properties even in the severe thermal environment (&gt;1500.degree. C.) of an internal combustion engine, as are the ceramics of this invention, but instead are materials used in refractory applications where strength and toughness are not critical, e.g., in furnace construction. In fact, it is well known that porosity detracts from toughness; cf. U.S. Pat. No. 4,543,345, column 4, at lines 55-6.
Whereas efforts to prepare porous alumina composites containing high strength alumina fibers have been successful, efforts to prepare fully dense alumina composites reinforced by alumina fibers have been unsuccessful. In the context of this application "fully dense" refers to material having at least 95% of the theoretical density of the ceramic material. The prior failures appear to arise from the necessity of sintering the composite to achieve densification. It is well known that an alumina composite needs to be heated under pressure at a temperature of at least about 1400.degree. C., and up to about 1900.degree. C., in order to provide a composite with essentially the full theoretical density of the ceramic, and that essentially 99% of the theoretical density is required to obtain the maximum toughness and strength. However, at the sintering temperatures necessary to achieve densification the alumina fibers dissolve into the surrounding aluminum oxide matrix, destroying the reinforcing fiber network. Thus, whereas the composite of EP 130,105 retains its fiber network because it is sintered at only 800.degree. C., were it processed to full density by, e.g., hot pressing at 1500.degree. C. there would be a loss of fibrous microstructure with resulting formation of a simple alumina body.
The problem which heretofore was an absolute barrier to the preparation of fully dense alumina fiber reinforced alumina composites was dissolution of the alumina fibers in the alumina matrix during the thermal treatment needed for densification. It seemed to us that the presence of a diffusion barrier between the alumina fibers and alumina matrix might be a solution to the aforementioned problem by preventing such dissolution and thereby allowing the presence of discrete high strength alumina fibers in an alumina composite. It appeared to us that encasing the alumina fibers in a sheath of zirconia would eliminate the dissolution of the fibers at high temperature into the surrounding ceramic matrix because zirconia and alumina are mutually insoluble. That is, zirconia does not dissolve in alumina nor does alumina dissolve in zirconia, and such a sheath or coating would maintain the integrity of alumina fibers in an alumina matrix during sintering. The formation of a more-or-less uniform coating of zirconia on the fiber surface should significantly impede degradation of the fiber within the matrix and thereby preserve the intended improvement of the material's fracture toughness and strength due to the fibers present. It is also possible that the zirconia coating, if present as the metastable tetragonal phase, could improve the toughness of the composite by conventional transformation mechanisms.
As is elaborated upon within, the zirconia sheath or coating described in this invention is not merely a physical deposition of discrete zirconia particles on the surface of alumina fibers, but rather chemical bonding of discrete oxygenated zirconium species to the surface of alumina. That is, the resulting fibers are surface-zirconated much as is described in U.S. Pat. No. 4,459,372. A key advantage to the coating methods used is the possibility of preparing coatings of controlled thickness by careful, surface-dominated reaction chemistry.